Graft rejection is initiated and sustained by alloreactive T lymphocytes which have matured in the thymus. During intrathymic ontogeny, T cell receptor (TCR) gene rearrangement creates an enormous number of T cells each bearing a unique TCR that is expressed on the cell surface in association with CD3 (TCR/CD3) complex. Immature TCR/CD3+ expressing cells are located in the thymic cortex, and express both the CD4 and CD8 accessory molecules (double positive thymocytes). Double positive thymocytes undergo selection processes which result in the death of at least 95% of the cells. These processes eliminate potentially autoreactive cells (clonal deletion) and deliver a signal required for continued maturation into mature T cells (positive selection). The precise mechanisms of both selection processes are unknown, but involve engagement of the TCR by MHC molecules located on the thymic epithelial cells or resident macrophages. TCR/CD3 stimulation of double positive thymocytes induces programmed cell death via DNA apoptosis (a likely mechanism of clonal deletion). Signal transduction via TCR/CD3 can be strongly enhanced by crosslinking either CD4 or CD8 to CD3, and we will first examine the hypothesis that this effect is translated into augmentation of programmed cell death. TCR/CD3 engagement can also lead to positive selection, but the differential response (i.e. clonal deletion or positive selection) of immature thymocytes to TCR engagement may depend on the absence or presence of costimuli delivered through accessory molecules. The CD28 molecule, present on double positive thymocytes, can provide a co-mitogenic signal along with TCR/CD3 costimulation. A natural ligand for CD28, BB-1 is present in thymic stromal tissue, and we will test the hypothesis that CD28 stimulation can deliver a second signal which rescues TCR/CD3 stimulated thymocytes from programmed cell death. Since engagement of the TCR/CD3 complex can lead to positive selection and clonal deletion, different protein product may be synthesized under the different stimuli. Alternatively, similar proteins may be produced, but their activity may be differentially regulated by phosphorylation. As the Phase II project, we plan to characterize the polypeptide(s) uniquely expressed or phosyphorylated under the stimuli leading to DNA apoptosis vs. maturation.Further definition of these events in T cell ontogeny should lead to a clearer understanding of the alloimmune response and may help develop ways to manipulate it.